Here are some questions and answers about how ICE operates:
WHAT CAN THEY DO?
The agency is in charge of arresting and deporting immigrants who lack legal status.
One common method of finding people who are known to be in the country illegally is for local jails to hold those who have been arrested on crimes past their release date. That allows ICE to look into their status. These are known as "detainers," but they have become increasingly unpopular. Some local governments complain that detainers put their officials at legal risk and that local authorities should not be doing the work of federal authorities.
ICE also arrests people the old-fashioned way, by tracking them down and showing up at their homes or workplaces. But limited staff and resources constrain their ability to make multiple large-scale arrests at a time.
WHAT HAVE THEY DONE?
Last fiscal year, ICE's Enforcement and Removal Operations unit arrested more than 158,500 immigrants in the country illegally, an 11% increase over the prior year and the highest number since 2014. The agency says 66% of those arrested were convicted criminals.
"I think what people forget is these operations go on on a regular basis," Houston Police Chief Art Acevedo said.
HOW DOES A SWEEP HAPPEN?
Authorities typically have a list of people they are targeting and administrative warrants giving them permission to detain those people for violating immigration law. They visit a targeted person's addresses, usually a home or workplace. They may interview relatives, neighbors, co-workers or managers.
ICE agents can also arrest people they discover to be in the U.S. illegally while searching for targets on their list. People who answer agents' questions about someone else sometimes end up arrested themselves. In one case in Houston last year, a young father of five was arrested in the parking lot of his apartment building after ICE agents asked him about people who lived nearby, then demanded his identification and eventually detained him.
These "collateral" arrests can comprise a large portion of the arrests in any operation. In a December 2017 operation in northern Kentucky, just five of the 22 arrests were people who were originally targeted, according to agency documents released under the Freedom of Information Act.
WHAT COMES NEXT?
Ten cities are expected to be targeted in raids starting this weekend, based on locations that have expedited court proceedings for recently arrived families. The cities are Atlanta, Baltimore, Chicago, Denver, Houston, Los Angeles, Miami, New York and San Francisco. Raids in New Orleans were put on hold due to severe weather.
ICE officials said this week that they sent about 2,000 letters in February to people in "family units" who had already received final orders to leave the country. The people who received those letters may be the targets of the enforcement operation.
Activists have been bracing for an increase of activity and have offered more training for immigrants to know their rights in case authorities show up.